Voice coil and diaphragm support for high frequency loud-speaker



E. M. VILLCHUR 3,033,945 COIL AND DIAPHRAGM SUPPORT FOR IGH FREQUENCYLOUD-SPEAKER Filed May 1, 1959 May 8, 1962 VOICE};

United btates Patent Q setts Filed May 1, 1959, Ser. No. 819,421 2Claims. (Cl. 179-1155) This invention relates to electrodyna-micspeakers for sound reproduction, and more particularly to speakers ofthe class referred to as tweeters intended primarily to reproduce thehigher frequencies of the audible range.

The flatness or uniformity of the response of a speaker over the rangeof frequencies for which it is designed, in the direction of its axis ofradiation, is frequently used as a measure of performance. The on-axisfrequency response curve, however, is not a complete measure of therealism of the reproduced sound. One of the well known deficiencies ofpresent speakers, especially those intended for the treble range, isprogressive beaming at higher frequencies; that is, the volume of soundtransmitted to positions away from the axis of the speaker, falls olfappreciably 'as the frequency increases. Therefore, the total energyradiated is attenuated as the frequency increases. Since a normallyreverberant room tends to integrate on-axis and off-axis sound, the neteffect is an attenuated treble response. Flat on-axis response combinedwith good dispersion, that is, uniform response over a reasonably largesolid angle, apparently produces the right ratio of direct to reflectedsound in the room to create a maximum effect of realism. Good horizontaland vertical dispersion not only permits the speaker to be listened toat off-axis positions, but also strongly influences the quality ofreproduction at positions on-axis with the speaker. Without suchdispersion much of the effect of the acoustics of the hall in which therecording was made is lost, and the listeners awareness of the speakeras an artificial sound source, of dimensions much smaller than anorchestra or choral group, is sharpened.

' The principal object of this invention is to produce a loudspeakerwhich has a substantially flat on-axis frequency response in the highfrequency range, combined with a reasonably flat total power responseand reasonably uniform dispersion both vertically and horizontally.

The speaker here disclosed consists, in general, of a relatively smalldirect-radiating diaphragm, preferably dome-shaped, having preferably acylindrical voice coil mounted directly on its rim, a magnet assemblyhaving an air gap in which the voice coil is disposed, and a compliantsuspension composed of elastic material, such as synthetic foam rubber,placed directly in the air gap, and adhesively secured both to the coiland to the sourroundin-g portion of the magnet assembly. Otheradvantages and novel features of the device will be apparent from thedescription which follows.

In the drawings illustrating the invention:

FIG. 1 is a front elevation of a loud speaker constructed according tothe invention;

FIG. 2 is a cross-section taken along line 2-2 of FIG. 1;

FIG. 3 is a front elevation of a modified form of the speaker; and

FIG. 4 is a front elevation of another modification of the speaker.

The magnet assembly, generally indicated by the numeral 10, is made upof a ring magnet 11, a top plate 12, a disk-shaped bottom plate 13 towhich is attached a cylindrical pole piece 14, and a spacer ring 15.Plate 12 has an inner rim 12a which defines a circular air gap. Thisplate also forms :a mounting flange for the ice speaker. The magnetassembly is held together by bolts 20 passing through the top and bottomplates.

A dome-shaped diaphragm 16 has a voice coil 17 attached to its rear rim16a, and disposed in the gap defined by rim 12a. The voice coil and rim12a define between them an air gap 18. The ends of the coil are carriedout to form leads 19. The diaphragm is made of a light, rigid material,such as phenolic impregnated fabric. For example, the diaphragm maybemade of two layers of cloth bonded together with their weaves at 45 toone, another. The voice coil is cylindrical and of the self-supportingtype, and is cemented to the diaphragm.

In the form shown in FIG. 1, plate 12 has several spaced notchm 21 inrim 12a. Four notches are here shown, but three, or more than four, maybe used. These notches are filled with pieces of elastic foam material22 (one of which is'shown removed in FIG. 2), which extend into the airgap 18 and are adhesively secured to the top plate 12 and to the voicecoil 17, forming the suspension for the diaphragm. A pad 23 of soundabsorbent material, such as glass wool, is placed between the end ofpole piece 14 and the diaphragm, and a packing 24 of similar materialfills the space inside the magnet, up to a screen 28 of non-magneticmesh material which is placed around the upper part of the pole piece tokeep the packing material out of the air gap.

Examples of a suitable material for the suspension pieces 22 arepolyurethane foam and foam latex. This material is put in a semi-solidor liquid uncured state, and then cured. Such a material is sufficientlyadhesive in itself to adhere to the voice coil and the core issufficiently compliant so that the resonant frequency of the diaphragmand voice coil assembly is near the lower end of the operating range forwhich the speaker is intended.

In the form of speaker shown in FIG. 3, the magnet assembly, diaphragm16, and voice coil construction are essentially the same as in the formpreviously described, except that plate 12 is replaced by a top plate 25with a continuous inner rim 25a. The entire air gap around the voicecoil is filled with a ring 26 of elastic material which forms thesuspension for the voice coil and diaphragm.

The speaker shown in FIG. 4 has the same magnet structure as that shownin FIG. 1, and the diaphragm and voice coil are the same as in the formspreviously described. The suspension consists of pieces 22 of elasticfoam material, as in FIG. 1, but the intervening spaces are filled witha highly compliant elastic material 27 such as butyl rubber which may beapplied in liquid form. The speaker may be mounted in a baffle in anystandard manner.

Some of the dimensions of the parts are shown some what exaggerated inthe drawings for clarity. For example, the diaphragm is very thin andthe voice coil is made of fine wire. The air gap 18 is in the order of.040 inch wide for a two inch diaphragm, and the notches areproportionately small.

To assemble the diaphragm in the speaker, the diaphragm is held in anappropriate fixture to center the voice coil in the gap. Pieces of thesuspension forming material in uncured form are placed in the gap andcured under heat, for example at about 300 F. The material, in curing,foams and adhers to the coil and top plate. An alternative method offorming the suspension is to secure pieces of suitable material, such ascured foam rubber, in the gap by cementing them to the voice coil andtop plate. To complete a speaker such as that shown in FIG. 4, butylrubber is poured into the open spaces in the air gap after the foampieces in the notches have been cured. The butyl rubber has a negligibleeffect on the compliance and serves mainly to seal the speaker againstdust.

aosaees The diaphragm is of light Weight and is made in relatively smallsizes, as the speaker is intended for the high frequency range.Preferably, the diaphragm is formed in the shape of a dome orapproximately a section of a sphere for maximum rigidity. Essentiallythe diaphragm operates as a direct-radiating rigid piston.

A diaphragm of two inch diameter is especially suitable for a range offrequencies of 1000 to 7500 cycles per second. The diaphragm and voicecoil have a mass. of approximately one gram and the compliance is suchas to produce a resonant frequency in the neighborhood of 1000 c.p.s.The measured on-axis response of one model of a two inch diaphragmspeaker in this range was flat within plus or minus 2 decibels, and onlyslight attenuation in ofi-axis response with increase in frequencyoccurred. Up to 5000 cycles per second the oft-axis response remainedwithin 2 decibels of the, onaxis response up to an angle of 45 in. anydirection from the axis. At 7500 cycles per second the oif axis attembation at 30 Was still less than 3 decibels, and at 45 1 within 6 or 7decibels. Above7500 c.p.s.,the off-axis attenuation increased somewhat,but was considerably less than that exhibited by high frequency speakersof conventional types.

For a higher frequency range a smaller diaphragm, for example a 1% inchdiameter diaphragm, may be used. One model of a speaker of this sizeexhibited negligible off-axis attenuation through an angle of 45 overthe 7500 to 10,000 c.p.s. range, and the on-axis response was fiatwithin plus or minus 1.5 decibels up to 20,000 c.p.s. At 15,000 c.p.s.the off-axis attenuation at 45 was within 6 decibels.

It is understood that the shape of the diaphragm, coil, and magnet maybe varied, for example the edge of the diaphragm, the coil, and gap maybe polygonal. The construction of the magnet assembly may also bevaried. For example, the pole piece structure may include a permanentmagnet. 1

i What is claimed is:

1. A loudspeaker comprising :a magnet structure having a rim defining agenerally circular air gap, said rim having circumferentially spacednotches, a generally circular diaphragm, a generally cylindrical voicecoil attached to said diaphragm and disposed in said gap in spacedrelationship to said rim, and pieces of elastic material disposed insaid notches and extending to said coil, said pieces being adhesivelysecured to said rim and said coil and constituting suspension means forsaid diaphragm.

2. A loud speaker comprising a magnet structure having a defining agenerally circular air gap, said rim having circumferentially spacednotches, a generally circular diaphragm, a generally cylindrical voicecoil attached to said diaphragm and disposed in said gap in spacedrelationship to said rim, and an elastic suspension means disposed insaidgap and adhesively secured to said rim and said voice coil, saidsuspension means comprising pieces of foam rubber-like material disposedin said notches and extending to said coil, and highly compliantrubber-like material bridging said gap between said pieces.

References Cited in the file of this patent UNITED STATES PATENTS

